This invention relates to a heat transfer system and particularly to one adapted for transferring heat from the exhaust gases of an internal combustion engine to its liquid coolant.
Most modern motor vehicles employ liquid cooling systems which remove waste engine heat to maintain the engine in a safe operating temperature range. In order to provide heated air for warming the vehicle passenger compartment and for defrosting, the heated coolant passes through a heat exchanger. The engine coolant system further stabilizes the operating temperature of the engine within a narrow range which is desirable in achieving performance, fuel economy, and exhaust emissions levels. The rate of heat transfer to the liquid coolant of the internal combustion engine coupled with the quantity and thermal characteristics of the coolant causes the temperature of the coolant to increase relatively slowly when the vehicle has been started after cold-soaking in low ambient temperature conditions. Accordingly, warm air for passenger compartment heating and windshield defrosting is not immeditely available. Consequently, the vehicle operator must often prestart the vehicle prior to using it in order to allow the coolant to reach a desired temperature. In addition to problems associated with occupant comfort and defrosting, the slow increase in coolant temperature adversely affects engine operation.
In view of the foregoing, there is a need for a system for increasing the rate at which liquid coolant of internal combustion engines is heated to a desired operating temperature.
One approach toward increasing the rate of temperature rise of engine coolant upon start-up is to transfer waste heat from the engine exhaust gases to the engine coolant. Heat pipe thermal transport systems enable heat energy to be transferred between a heat source and a remote heat sink with high efficiency, and therefore, are excellent candidates for such applications. Heat pipes are comprised of an enclosed vessel having separated evaporator and condenser sections. An external heat source supplies thermal energy to the evaporator section and a heat sink extracts heat from the condenser section. The heat pipe has a hollow interior cavity which is filled or lined with a wick of porous material. The interior of the heat pipe is charged with a heat transfer medium which vaporizes in the evaporator portion, and is transferred with its latent heat of evaporation to the condenser section where it condenses on the cooler surfaces, thus giving up its latent heat. The condensed medium is transferred back to the evaporator portion through the porous wick by capillary action and/or gravity.
Due to structural considerations, there are limits on the temperature ranges through which a heat pipe may operate. Temperatures much higher than the desired operating range of the heat pipe causes internal fluid pressures to increase to a level which could causes structural failure of the heat pipe. This limitation poses a significant design obstacle in designing a heat pipe system for transferring waste exhaust gas heat to engine coolant since exhaust gas temperatures vary widely, for example, over a range of 250.degree. C. to 700.degree. C.
One means of handling such temperature extremes is by choosing a working fluid which has a vapor pressure which does not exceed the upper pressure limit of the heat pipe at the highest temperature to which the heat tube would be subjected. Such a working medium, however, would have a very low vapor density at lower temperatures which would require an excessively large core diameter to provide the desirable thermal transport rate.
Another means of overcoming the previously described design challenges for heat pipe type systems would be controlling the heat input to the heat pipe evaporator portion by diverting portions of engine exhaust gas out of contact with the evaporator. This approach, however, would require the use of a mechanical valve in the exhaust gas stream. Such valves are undesirable due to reliability, cost, and other considerations.
Another approach for rapidly heating coolant is to allow the exhaust stream to freely deliver heat to a heat pipe evaporator portion and then dissipate any excess heat. This approach, however, is not viable due to the requirement to dissipate extremely high amounts of heat.
The system according to this invention achieves the above-mentioned desirable features and overcomes the shortcomings of the approaches described above by permitting portions of the heat pipe evaporator to boil dry as exhaust gas temperature increase. Such portions will reach exhaust gas temperature and will not thereafter participate in the heat transfer through the heat pipe. Accordingly, heat input through the heat pipe remains at a substantially constant level. This system provides a means for varying the amount of heat transfer medium which participates in the heat exchange process in the heat pipe by storing a quantity of the working fluid in condensated form when it is not needed. When exhaust temperatures increase or the rate of heat transfer to the coolant decreases, larger proportions of condensed working fluid are stored. Conversely, when exhaust gas temperatures are low and heat transfer rates to the coolant are high, a larger proportion (or all) of the working fluid is permitted to transfer heat between the evaporator and condenser.